This invention relates to sewing machines and particularly to an improved work piece feeding mechanism of the type that imparts feeding action to both the top and bottom surfaces of the work piece in timed relationship with the stitch forming mechanism.
For sewing operations involving fabrics, such as relatively thick denim used in jeans that is difficult to feed as well as other operations where the control of the fabric layers relative to each other is critical, feeding devices that impart a feeding action to both the upper and lower surface of the work piece are used.
Machines of this type find particular application when joining the legs of jeans. In this operation an abrupt change from four plies to sixteen plies of material is encountered. The superior ability to accurately control and feed uniquely qualifies machines of this type to successfully accomplish this task.
The lower surface feeder includes a presser foot, which is located above the work piece, and feed dogs which are located below the work piece. The work piece is grasped by the teeth of the feed dogs, in engagement with the lower surface of the work piece, and pulled horizontally along the lower smooth surface of the presser foot.
The drive mechanism for the upper surface roller feeder, which is a critical element of this device, includes a roller that is biased downwardly toward the throat plate. The aggressive surface of the roller engages the upper surface of the work piece and when rotated pulls the work piece along the smooth upper surface of the throat plate.
The upper surface roller feeder must be synchronized with the lower surface feeder and with the needle drive and must be driven intermittently so that movement is imparted to the work piece only when the needle is not penetrating the work piece. The needle and looper mechanisms are also carefully synchronized. The roller for the upper feeder and the presser foot for the lower feeder must be free to move vertically in response to changes in the work piece thickness or contour. However, the roller for the upper feeder and the presser foot for the lower feeder contact different areas of the upper surface of the work piece at a given time. For this reason, the roller for the upper feeder and the presser foot must be able to move vertically independently of each other. When the guide mechanisms for the upper feeder roller and the presser foot are interdependent, these elements can bind as a result of wear to the guide mechanisms, which locks the elements together and prevents independent movement.
The Union Special Corporation, class 35800 Feed-Off-The-Arm sewing machine includes a mechanism for imparting a feeding action to both the upper and lower surfaces of the work piece. The Union Special Corporation, class 35800 sewing machine is disclosed in Union Special Catalog No. 95DM. The presser foot has an upper guide mechanism that is located at the upper end of the presser foot bar within the head of the sewing machine. This upper guide mechanism functions solely to guide the presser foot. A second or lower guide system for the presser foot and the feed roller is located at the lower end of the presser foot bar. This lower guide system includes sliding surfaces on the presser foot mechanism and on the feed roller mechanism that permit independent movement of these elements. The presser foot mounting fork, which is a cast component carried by the lower end of the presser foot bar, has a first set of parallel vertically oriented machined surfaces. The feed roller frame, which is also a cast element has a second set of parallel vertical oriented machined surfaces. The engagement of the first and second sets of machined surfaces functions to maintain proper alignment of the upper surface roller feeder and the presser foot. However, when these sliding surfaces wear, the elements can twist relative to each other and under some conditions cause the elements to bind and thus not operate independently. When this occurs the operation of the machine is adversely affected and the machine must be serviced. Since the sliding surfaces are formed on structural parts of the sewing machine, when the surfaces have worn such that machine operation is no longer acceptable the entire elements must be removed and replaced. This is a time consuming and expensive repair operation which idles the sewing machine and often the operator for prolonged periods of time. In this machine the drive for the feed roller mechanism is derived from the main drive shaft through an eccentric and pitman that oscillates a bellcrank which is connected to an input clutch drive member through a connecting bar. The clutch imparts a drive motion, in one direction, to the drive mechanism for the upper surface feeder.